Monitoring operating conditions in commercial or industrial applications is often crucial to maintaining proper and safe operation of machinery. Material temperature can affect the performance of machines, apparatus, or systems. Continuous operation or repeated use often elevates the temperature of machines or apparatus, and uncontrolled or overly elevated temperatures may impair their operation.
The military has similar concerns as those of commercial or industrial entities regarding material conditions such as temperature. In particular, the military recognizes that operating conditions, such as temperature, affect the performance and the safe and proper operation of structures subject to extreme temperature variations. Such structures include outer body surfaces of high-speed vehicles and inner surfaces of chambered structures. For example, an outer structure of a vehicle in hypersonic flight will undergo surface temperature changes due to frictional heating when traveling at high Mach numbers. A chambered structure that can undergo extreme temperature changes is a combustion chamber of a liquid rocket motor. In either case, the structures are virtually inaccessible for direct temperature measurements during operation.